1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus and method for an internal combustion engine and, more particularly, to an air-fuel ratio control apparatus for an internal combustion engine that detects the air-fuel ratio of an air-fuel mixture supplied into the engine and performs the feedback control of the concentration of fuel in the mixture on the basis of the detected air-fuel ratio so that the air-fuel ratio becomes equal to the theoretical air-fuel ratio.
2. Description of the Related Art
A technology for controlling the air-fuel ratio of air-fuel mixture to the theoretical air-fuel ratio with high precision has been developed in relation to the emission control of an internal combustion engine using a catalyst. In this technology, a sensor, such as an oxygen sensor or the like, that is able to detect a physical quantity that indicates an air-fuel ratio is disposed in an exhaust passage of the internal combustion engine. The technology detects the air-fuel ratio based on components of exhaust gas by using the sensor, and reflects a result of the detection in an air-fuel ratio feedback adjustment coefficient. Based on the air-fuel ratio feedback adjustment coefficient and the amount of intake air, the technology calculates an amount of fuel to be injected. Thus, the amount of fuel supplied corresponding to the amount of intake air is adjusted by using an air-fuel ratio feedback adjustment coefficient so that air-fuel ratio control can be performed with good precision.
For example, if the air-fuel ratio based on components of exhaust gas is on a fuel-lean side of the theoretical air-fuel ratio, the value of the air-fuel ratio feedback adjustment coefficient becomes greater, and the amount of fuel supplied is increased. Therefore, the fuel concentration is adjusted to a higher side so that the air-fuel ratio is brought closer to the theoretical air-fuel ratio. If the air-fuel ratio based on components of exhaust gas is on a fuel-rich side of the theoretical air-fuel ratio, the value of the air-fuel ratio feedback adjustment coefficient becomes smaller, and the amount of fuel supplied is decreased. Therefore, the fuel concentration is adjusted to a lower side so that the air-fuel ratio is brought closer to the theoretical air-fuel ratio. In this manner, the air-fuel ratio is adjusted to the theoretical air-fuel ratio with good precision.
Another technology, disclosed in, for example, Japanese Patent Application Laid-Open No. HEI 4-203446, performs feedback control of the air-fuel ratio by increasing or decreasing the amount of intake air, instead of the air-fuel ratio feedback control that adjusts the amount of fuel supplied. This technology increases or decreases the amount of intake air based on the value of the air-fuel ratio feedback adjustment coefficient, so as to control the air-fuel ratio to the theoretical air-fuel ratio with good precision.
However, the above-described air-fuel ratio feedback control technologies have a problem of fluctuation of torque produced by the internal combustion engine.
In the air-fuel ratio control technology based on the adjustment of the amount of fuel supplied, a state in which the amount of fuel supplied is increased to shift the concentration of fuel in the air-fuel mixture from a low value to a high value, and a state in which the amount of fuel supplied is decreased to shift the concentration of fuel in the air-fuel mixture from a high value to a low value are alternated. The torque produced by the internal combustion engine varies to a relatively great extent between the two states. That is, the torque is smaller when decreasing the amount of fuel supplied than when increasing the amount of fuel supplied. Therefore, fluctuation of the torque produced by the internal combustion engine is caused by the air-fuel ratio feedback control.
In the air-fuel ratio control technology based on the adjustment of the amount of intake air, a state in which the amount of intake air is increased to shift the concentration of fuel in the air-fuel mixture from a high value to a low value, and a state in which the amount of intake air is decreased to shift the concentration of fuel in the mixture from a low value to a high value are alternated in order to bring the air-fuel ratio closer to the theoretical air-fuel ratio. In this case, too, the torque produced by the internal combustion engine decreases during the state of decreasing the amount of intake air. The torque difference between the two states is relatively great. Therefore, fluctuation of the torque produced by the internal combustion engine is caused by the air-fuel ratio feedback control, as in the technology based on the adjustment of the amount of fuel supplied.
Therefore, in either one of the air-fuel ratio feedback control technologies, the drivability becomes insufficient in some cases.